1. Field of the Invention
This invention relates to the gravel packing of oil wells, gas wells, and similar boreholes and is particularly concerned with an improved method of gravel packing.
2. Description of the Prior Art
Many hydrocarbon reservoirs are poorly consolidated and therefore production of these reservoirs often requires the application of sand control techniques. Gravel packing is widely used for controlling the production of sand from oil wells, gas wells, and similar boreholes. This normally involves the injection of a fluid suspension of sand, gravel, or similar particles into the well to form a permeable bed of solids adjacent the formation. Any formation fines carried by the produced fluids are trapped and deposited at the periphery of the gravel pack, permitting clean fluids to be produced.
The most common type of gravel packed completion involves placing a screen or slotted liner in the wellbore adjacent the incompetent formation and introducing a fluid suspension of the gravel packing material into the wellbore so that the bed of gravel is formed around the screen or liner. The use of screens or liners is generally accompanied by certain undersirable features, including a steady decline in the productivity of the well as the screen is gradually plugged. Such devices frequently have a relatively short life span and their removal complicates workovers. The formation of a single channel through the gravel as a result of shifting and settling may be sufficient to cause screen plugging and failure, requiring a costly and time-consuming workover to resume production.
Another type of gravel pack is the consolidated gravel pack. This technique involves introducing into the wellbore around the screen or liner a solution containing a plastic consolidating agent in addition to the gravel packing material. Once the gravel bed has cured, a high permeability, consolidated pack is formed which mechanically restrains formation sand. This technique is not widely used since any plugging of the screen or liner results in an extremely difficult work-over operation.
These difficulties have led to a third broad type of gravel pack wherein a consolidated gravel pack is formed in the wellbore annulus (and typically in an annular cavity outside casing as well) adjacent the formation without the use of a screen or liner. Once fully consolidated, the consolidated gravel bed inside the casing is either fully or partially drilled out. An example of this technique is described in U.S. Pat. No. 3,867,986. However, this technique can present difficulties since drilling can cause cracks to form in the gravel pack; also, drilling tends to form solids which can plug the consolidated gravel pack. In addition, the heat generated by drilling can damage the inner cylindrical wall of the plastic consolidated gravel pack, resulting in reduced rates of production.